Rethinking Parasite Control: Part 1

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A Fresh Look at an Old Problem

by Dave Sauter, DVM


Widespread use of outdated de-worming practices that were developed 30 or 40 years ago is leading to the development of “super worms” resistant to currently available drugs. This is not a new revelation, but the problem is growing and getting more attention. With no new products likely in the foreseeable future, it is time to have a fresh look at the subject of parasite control in horses.

Main equine internal parasites

There are hundreds of species of internal parasites. Here is a short list of the most clinically significant.

Strongyles, large and small: Large strongyles have been virtually eliminated, but small strongyles have become an increasing problem. An individual horse can harbor between 30,000 and 100,000 adults in the lumen of the intestine where they rob the horse of nutrients. Each adult can produce 100,000 eggs per day. Small strongyle larvae migrate through the intestinal wall where they cause more serious clinical problems for the horse, such as diarrhea, weight loss and colic. Tapeworms: Long considered insignificant, we have learned that they are more prevalent than previously thought and responsible for a large percentage of spasmodic colics. Ascarids: These are large round worms. Most adults develop immunity to them, but they can be highly destructive and lethal to foals and weanlings. The larvae migrate through the abdominal cavity, liver and lungs. Heavy infestation can result in respiratory symptoms, rough hair coat, pot belly, weight loss, and colic. Bots: Bots are not worms but insects whose larval stages are dependent on developing inside the horse’s stomach. The larva sheds the following summer, pupates, and becomes an adult botfly.

Pinworms: These worms are not a life threatening parasite, but definitely can be a nuisance. The adult lives in the rectum, exits the anus to lay her eggs on the skin causing irritation and itchiness.

Background of parasite control programs

During the 1960’s benzimidazole de-wormers were developed and revolutionized de-worming practices. They were safe, inexpensive, readily available (over-the-counter), easy to use, and very effective, particularly against the large strongyle—Strongylus vulgaris—which, at the time, was the leading cause of colic in horses. Benzimidazole use, including the recommendation to de-worm every 8 weeks, was widely adopted. With it and other paste de-wormers developed through the 70’s and 80’s, large strongyles were virtually eliminated as a problem and represented a huge gain for the health of horses.  As the paste de-wormers developed, the need for the far more costly and involved tube de-worming fell by the wayside.    

By the 1980’s, it became clear that as we solved one problem a new one was emerging. Frequent de-worming inadvertently selected for genetic “super worms” that are resistant to de-wormers. Frequent exposure to the de-wormer drugs kills off sensitive parasites, but not the resistant ones. As time goes by, a greater and greater proportion of resistant parasites develop. 

There are currently three major classes of de-wormers: benzimidazoles – already mentioned above such as Panacur™ and Safeguard™; pyrantel salts such as Strongid™; macrolides – ivermectin, which has been around about 25 years; and, the newest drug, moxidectin.

For many years we have known about strongyle resistance problems to benzimidazoles. More recently, studies have shown resistance developing to pyrantel. Studies have also shown problems of ascarid resistance to ivermectin. As far as we know, small strongyle resistance to ivermectin and moxidectin has not yet occurred in horses. In small ruminants (such as sheep and goats), there is documented internal parasites resistance to ivermectin.

There is enormous cost and a very lengthy process for the discovery and approval of new drugs. We have been informed there is no reason to expect any new equine de-wormer products in the foreseeable future. Therefore, it is imperative to use what we have available conservatively in an effort to preserve the effectiveness of our current arsenal.    

Part Two of this series will talk about why, when developing an effective and sustainable de-worming program, it is important to treat each horse as an individual. 


Published September 2012 Issue

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